In a conventional rotary veneer lathe, there is provided a pair of powered spindles which are rotatable synchronously with one another and arranged so as to hold a peeler log at its opposite ends by way of chucks mounted at the ends of such spindles and to drive the log to rotate in cutting operation, and a veneer knife fixedly mounted on a carriage movable along guide ways by lead screws or hydraulic cylinders for feeding the veneer knife into the log periphery at an appropriate rate for producing a veneer sheet with a predetermined thickness. The thickness of a veneer sheet to be peeled in a veneer lathe is determined by the amount of feed of the knife cutting into the peeler log for each turn of the log. For the purpose of controlling the feed rate of the cutting knife supported on the carriage, a feed mechanism for the knife carriage is operatively connected to at least either one of the synchronously driven spindles to detect or monitor the log rotation thereby to determine the carriage feed in conjunction with the rotation of that connected spindle. In such an arrangement, insofar as the log rotation is maintained substantially the same as the rotation of the spindles which drive the log, normal and safe veneer peeling can be accomplished with no fear of producing defective veneer sheet having variation in thickness or of a possible damage, as will be described hereinafter, to the peeler log and/or the veneer lathe per se. Since it is practically difficult to maintain such a condition at all times during lathe operation for logs of all possible qualities, it is unavoidable to have the log rotated at a speed other than that of the spindles.
It is commonly observed during veneer peeling operation on a veneer lathe that the strength of a log being rotated by the spindles is reduced at the location of its engagement with the spindles to such an extent that it cannot resist any more the driving force exerted by the spindles and consequently part of the log is broken gradually or even suddenly, with the result that the log rotation fails to coincide with the spindle rotation. If the spindles are allowed to continue running without application of brake thereto at a proper time, it may result in such a situation that the spindles are continued to run in slipping engagement with the log ends in spite that the log has already been brought out of driving engagement with the spindles and hence substantially stopped. In the lathe of the above conventional arrangement wherein the knife carriage is moved in conjunction with the spindle rotation, the knife carriage feeding will be continued to cause the knife to advance against the log which may be then already at a stop. The result will be excessive and damaging pressure taking place between the peeler log and the knife carriage, which may cause the lathe parts including spindle bearings, cutting knife, etc. and/or the log to be damaged seriously.
As it is known to many veneer or plywood manufacturers, a veneer lathe having a log drive system of new type which is designed to apply turning torque to a log from its outer periphery for driving the log or for assisting in the log driving, as proposed, e.g., by U.S. Pat. Nos. 4,221,247 and 4,269,243, has been put in practical use and is gaining its popularity on the market for its inherent advantages.
In a veneer lathe equipped with such a peripheral drive system, it is usually necessary to drive positively a peeler log initially by using a pair of powered spindles and then to engage drive members of the peripheral drive system with the log for permitting smooth starting of cutting operation. It is also required that the log should be driven by the spindles during roundup of the log at an early stage of cutting, or initial cutting until the log becomes substantially cylindrical, during which the drive members are set to engage the outermost portions of irregularities on the log periphery and, therefore, they may be positioned out of driving engagement with respect to the rotating log. When the driving force or torque is supplied during normal veneer peeling operation mainly from the spindles with the peripheral drive system acting to assist in the log driving, the log ends in engagement with the spindles are naturally susceptible to the damage as mentioned above. When the peripheral drive system is used as the main log drive with the spindles as an auxiliary, on the other hand, the potential speed of the spindles in terms of the log peripheral speed is usually set slightly higher than the speed of the peripheral drive so as to avoid a situation in which the spindle rotation may offer a resistance against the log rotating by the main drive. In the latter allocation of the driving torque, the log ends are subjected to damaging force exerted by the spindles, too.